Refrigeration systems are well known which employ a single refrigerant, for example, CFC refrigerants such as R-12 and HCFC refrigerants such as R-22. These refrigerants, however, have serious environmental drawbacks and are being replaced by refrigerants of the HFC type such as R-32, R-125 and R-134a in different combinations.
The individual HFC refrigerants have diverse characteristics, as shown in the following table:
LATENT HEAT BOILING HEAT CONDENSER EVAPORATOR TRANSFER FLAM- DENSITY POINT (h.sub.fg) PRESSURE PRESSURE CHARACT. ABILITY R-32 Light Low Large High High Good Yes R-125 Heavy Low Small High High Medium No R-134a Medium High Medium Low Low Poor No
In many refrigeration systems, the following characteristics are preferred:
Density--heavy PA1 Boiling Point--low at evaporator and high at condenser PA1 Latent Heat--large PA1 Condenser Pressure--low PA1 Evaporator Pressure--high PA1 Heat Transfer--good PA1 Flamability--no PA1 v=specific volume=1/density; and PA1 P=pressure. PA1 1. Promote liquification in the first condenser by recirculating R-134a rich liquid into the first condenser section; PA1 2. Pass vapor to the second condenser section which is rich in R-32 and R-125; PA1 3. Follow thermal load variation by controlling the amount of recirculating R-134a. PA1 1. The use of an inflammable fluid; PA1 2. A large heat capacity at evaporator; PA1 3. A lower condenser pressure; PA1 4. A higher vapor pressure in the evaporator, producing a lower specific volume v in the evaporator, thus reducing compressor work .intg.vdP.
In the above, h.sub.fg is the enthalpy difference between 100% vapor and 100% liquid.
R-32 is a preferred refrigerant because of its high latent heat and high evaporator pressure which reduces the compressor work and thus the compressor size. That is, the compressor work W.sub.COMPRESSOR is defined as: EQU W.sub.COMPRESSOR =.intg.vdP
where
In a typical system, as evaporator pressure increases, the pressure change in the compressor is reduced, thus reducing the compressor work.
While R-32 has the best thermal characteristics, it is more flammable than the others, and carries with it the danger of fire. Consequently, R-32 is commonly mixed with non-flammable fluids such as R-125 and R-134a to reduce the fire danger.
Currently available mixture refrigerants include R-407c and R-410a. The former (R-407c) is one of the R-407 series refrigerants, which include R-407a, R-407b, R-407c, etc. The R-407 series is made of three refrigerants R-32, R-125 and R-134a. The last letter in the designation of R-407 indicates different composition ratios of R-32, R-125 and R-134a. For example, R-407c is made of R-32, R-125 and R-134a at a ratio of 23:25:52 based on mass. Similarly, R-410a is one of the R-410 series refrigerants which are made of two refrigerants R-32 and R-125. The last letter "a" in R-410a indicates that a composition ratio of R-32 and R-125 is 50:50 by mass. Depending on the composition ratio, the last letter can vary.
Several new HFC type refrigerants such as R-134a, R-407c and R-410a are known in attempts to get the best trade-off of flammability versus thermal efficiency. The first R-134a has replaced R-12 for automotive air conditioners, refrigerators and large chillers. This refrigerant has relatively poor heat transfer characteristics but in a typical system produces a pressure of about 8 atm at the evaporator and 16 atm at the condenser. Thus, the relatively small .DELTA.P at the compressor produces excellent efficiency. Therefore, this refrigerant has replaced R-12 for many applications, despite its poor heat transfer characteristics.
A second HFC type refrigerant is R-407c, which is a mixture of R-32, R-125 and R-134a in proportions of 23:25:52 respectively. This mixture, however, produces only about 6 atm at the evaporator and 20 atm at the condenser (like R-22) and has poor heat transfer characteristics due to the high proportion of R-134a.
A third HFC type refrigerant is R-410a, which is a mixture of R-32 and R-125 in a ratio of 50:50 respectively. This mixture, however, produces about 12 atm at the evaporator, but 30 atm at the condenser and requires a large compressor and compressor work.
It would be very desirable to provide a novel refrigeration system which would permit the use of an inflammable mixture of refrigerants, a reduced condenser pressure and an increased evaporator pressure; and which takes the best advantage of the properties of the individual fluids of the mixture.